Suicide has a devastating impact on victims, their extended families, and public health in the United States, as well as throughout the world. Few treatments have been shown to reduce risk. An exception, supported by extensive clinical evidence, is that lithium is effective in reducing the risk of both attempted and completed suicide. However, the mechanisms underlying lithium's antisuicidal actions are not yet known, limiting the development of improved prevention approaches. We intend to use the mouse as a model organism to elucidate molecular pathways by which lithium interacts with biological and behavioral factors associated with suicide in humans. However, rather than attempting the infeasible task of modeling suicide in mice, we will focus on approaches that assess mouse behavior in tests relevant to well validated endophenotypes (deconstructed components of complex behavioral phenotypes) associated with suicide including aggression and impulsivity. These endophenotypes will be used in combination with human genetic, biochemical, and pharmacological findings in suicide research to provide construct-valid animal models. Toward this end, clinical studies have implicated polymorphisms in a number of genes, including neuronal nitric oxide synthase (NOS1), with measures of impulsivity, aggression, and suicide. Similarly, the results of extensive research have implicated deficits in serotonin (5-HT) neurotransmission in the etiology of suicidal behavior as well as increased impulsivity and aggression. Data from preclinical and human genetic studies indicate that lithium may exert some of its mood stabilizing effects through inhibition of the enzyme glycogen synthase kinase-3! (GSK-3!). Intriguingly, emerging basic science evidence links NOS1 function, 5-HT neurotransmission, and GSK-3! activity suggesting that they may be causally linked in the pathophysiological processes relevant to the etiology and treatment of psychiatric diseases such as suicide, where impulsivity and aggression play a role. Thus, our Specific Aims are to: 1) Identify the effects of lithium on behavior in mice with genetically- and pharmacologically-induced decreases in 5-HT levels; 2) Identify the effects of lithium on behavior in mice with genetically- and pharmacologically-mediated deficiencies in nitric oxide synthase 1 (NOS1) activity; 3) Evaluate the role of glycogen synthase kinase-3!, a direct target of lithium, in modifying behaviors mediated by decreased 5-HT and NOS1 function. These studies will capitalize on current knowledge of lithium pharmacology and use mouse genetic knockouts and pharmacological approaches to dissect the molecular and neurobiological mechanisms whereby lithium may modify impulsive and aggressive behavior as well identify points of interaction between lithium and biological markers known to be associated with suicide.